At present, liquid crystal display elements have been widely used in the field of flat-panel displays. Among these, the TN (Twisted Nematic) type display elements, which have advantages such as low driving voltage and low power consumption, have been most widely used as high-quality display elements.
However, the response speed of the TN type display elements is inferior to a great degree, as compared with the response speed of display elements of the light-emission type, such as cathode ray tube, electro-luminescent, or plasma displays. Further, the TN type display elements that have a twisted angle set in the range of 180.degree. to 270.degree., that is, the so-called STN (Super Twisted Nematic) type display elements, have been developed, resulting in a drastic increase in the display capacity. However, these STN type display elements still have a limitation in improvements in the response speed.
Recently, display elements, in which each pixel of the TN-type display element is provided with a switching element, have been introduced in the market. Since most of these display elements have thin-film transistors (TFT) as the switching elements, they are referred to as the TFT-type display element, and expected to have future developments as liquid crystal display elements suitable for high-density, large-capacity and full-color display.
However, since the TFT-type display elements are manufactured by adopting semiconductor-manufacturing techniques, the available screen size is limited to as small as ten and several inches, and the cost of production is expensive. Moreover, the time-dividing capability of the TFT-type display element is said to be limited to approximately 1000 lines.
Here, a display method using ferroelectric liquid crystal has the possibility of achieving large screens not less than ten and several inches with reduced production costs, an advantage that have not been achieved by TFT-type display elements. This possibility has been suggested in "Applied Physics Letters" 36, (1980) p.899 written by N. A. Clark and S. T. Lagerwall.
The above-mentioned display method, which utilizes a chiral smectic phase, such as a chiral smectic C phase, that exhibits a ferroelectric property, is generally referred to as the surface stabilized ferroelectric liquid crystal (hereinafter, referred to as SSFLC) display method. The SSFLC display method has been studied by electric appliance makers and material makers so as to introduce it to their products, and improvements in its characteristics have been made for this purpose.
The main advantages of ferroelectric liquid crystal (hereinafter, referred to as FLC) are high-speed responsibility, good memory property and wide viewing angle. Since such advantages suggest that the SSFLC method will provide displays with large capacity, the SSFLC method is highly prospective in its practical use.
However, the main disadvantage of the SSFLC method is that since SSFLC exhibits a bistable property wherein it is basically stabilized in two orientation directions, it is difficult to achieve gray scale display.
As illustrated in FIGS. 11(a) through 11(c), in the common FLC, as the width of a pulse voltage to be applied to a pixel is increased from the bright state, domains appear and the domains change to an intermediate state 2 through an intermediate state 1. In this change, the domains expand abruptly in response to the change in the pulse width, and become larger individually. Further, since the threshold-value characteristic is constant irrespective of locations, the areas of the domains can not be desirably controlled due to factors, such as the surface state of the liquid crystal cell, the effective electric field to be applied to the liquid crystal cell and the nonuniformity in temperature. As a result, the domains expand irregularly. In this manner, the common FLC is not suitable for gray scale display.
In order to solve this problem, several methods for achieving gray scale display by utilizing FLC have been proposed.
For example, Japanese Laid-Open Patent Publication 194635/1994 (Tokukaihei 6-194635) discloses a method for forming a structural element wherein non-reactive chiral liquid crystal molecules are captured in an anisotropic three-dimensional micro structural element that has been made of a polymeric substance. With this arrangement, minute adjacent domains, which have polarization directions opposing to each other, can be stabilized by the micro structural element. Thus, the gray level can be maintained even in a non-electric field state.
As illustrated in FIGS. 12(a) through 12(c), in a liquid crystal display element containing the above-mentioned three-dimensional micro structural elements, as the width of a pulse voltage to be applied to a pixel is increased from the bright state, domains appear and the domains change to an intermediate state 2 through an intermediate state 1. In this change, the domains gradually expand in response to the change in the pulse width, and the number of domains gradually increases. Here, FIGS. 12(a) through 12(c) show the change of domains within approximately 1 mm square.
Moreover, Japanese Laid-Open Patent Publication 248489/1995 (Tokukaihei 7-248489) discloses the technique for forming minute domains by use of three-dimensional meshed synthetic resins composed of polymers. In this technique, composite elements (micro polymer) between the FLC and the resin are obtained by subjecting the prepolymer to photopolymerization under a temperature at which the FLC exhibits a nematic phase, and through this process, a striped domain structure extending in the rubbing direction is formed. Then, the area of a region to be switched is controlled by utilizing the fact that the respective domains have different threshold-value characteristics; thus, gray scale display can be obtained.
However, when the three-dimensional micro structural elements, obtained by the former technique, are adopted, the sizes of domains to be formed in the liquid crystal are randomly determined. For this reason, it is difficult to control the uniformity of the domain size, and the areas of domains cannot be desirably controlled upon application of the pulse voltage. Further, since each of the domains is larger than the actual size of a pixel (approximately, 0.3 mm square), it is practically impossible to display gray shades by using the above-mentioned technique.
Moreover, since the polymer molecules themselves, which constitute the three-dimensional (anisotropic) micro structural elements, are orientated, it is considered that the three-dimensional micro structural element is a structural element wherein it is orientated together with the liquid crystal structural element or liquid crystal. For this reason, when micro structural elements with a high concentration (5 to 60% by weight) are formed in the liquid crystal layer, a great interaction occurs between the micro structural elements and the FLC molecules. Then, this interaction gives adverse effects on the switching operation to a great degree, thereby resulting in difficulty in the high-speed driving of the FLC.
As described above, the former technique is not suitable for high-precision display. Furthermore, the presence of three-dimensional micro structural elements with a higher concentration not only causes light scattering, but also makes a disturbance in the uniform oriented state of the FLC molecules, thereby reducing the contrast of display images.
On the other hand, in the latter technique, since each of the long narrow striped domains has a size of approximately 100 .mu.m, it is too large to actually apply to matrix-type display elements. Further, since the striped domains exist inside a pixel at random, it is difficult to obtain the same gray level in a plurality of regions. Therefore, it is also difficult for the latter technique to actually provide gray scale display.
As described above, none of the above-mentioned techniques provide a structure that is capable of controlling the areas of switching domains uniformly over a wide range; therefore, it is impossible to display gray shades over a wide range, even though gray shades can be locally obtained.